The invention relates to a method for correcting a synchronization error in a digital radio system comprising at least one base-station communicating with terminals in its coverage area, and a mobile telephone exchange communicating with the base station and controlling the operation of the base stations, and that information to be transmitted is coded and decoded in a transcoder unit into a form suitable for the transmission, and that the base station sends information frames to the transcoder at a certain pace, and that the transcoder sends information frames to the base station at a certain pace.
A typical cellular radio system comprises a fixed base station network comprising one or several base stations with which a subscriber terminal communicates. In addition, the cellular radio system comprises, for example, a base station controller controlling the base stations, and a mobile telephone exchange.
The requirements set on data transmission systems continue to increase. This applies particularly to wireless digital data transmission systems, such as cellular radio systems from which an increasingly wider range of services, such as different data services, are required.
Transmission of information on a channel having a restricted band-width is always a kind of compromise between the quality of transmission and the data transmission rate observed by a user. In order to optimize transmission of information, the information to be transmitted is coded in different manners in different phases on a transmission path.
In the fixed network, higher transmission rates are usually employed than in radio systems. For that reason, transmission rate adaptation is required between the fixed network and a radio system. A transcoder unit is employed in the transmission rate adaptation. The transcoder unit, i.e. TRAU (Transcoder/Rate Adaptor Unit) codes for example a signal of 64 kbit/s coming from the direction of the fixed network into a signal with a lower transmission rate and suitable for a radio network. Adapting the transmission rate of the signal enables optimization of transmission line traffic capacity. The TRAU functions as a source coder and reduces the bandwidth between the base station and the subscriber terminal. Naturally, a corresponding reverse coding is also performed in the other transmission direction.
The TRAU is placed for example in connection with a base station controller or a mobile telephone exchange. The TRAU and the base station communicate by sending and receiving TRAU frames. The frames include for example coded speech, control information signalling, or other data.
The present invention can be applied particularly to the GSM radio system when the data transmitted by the frames between the transcoder and the base station has a transmission rate of 7.2 or 14.4 kbit/s.
The base station and the transcoder send frames to each other. This transmission occurs synchronically, i.e. the apparatuses must be in synchronization. In a prior art system, the base station functions as a controller with respect to synchronization. The base station sends frames to the transcoder in the uplink transmission direction. The transcoder synchronizes with a signal of the base station and sends frames to the base station in the downlink transmission direction in synchronization with the uplink direction. If a disturbance occurs in the transcoder, and the synchronization of the frames sent to the base station is disturbed, the base station sends to the transcoder special synchronization frames by means of which the transcoder is resynchronized.
A problem in the above described arrangement is that the actual information frames are replaced with the synchronization frames, i.e. transmission of information is broken for a period of several frames. Since the base station functions as the controller of synchronization, only the base station can start resynchronization by sending synchronization frames.
An object of the invention is to provide a method and an system so as to solve the above mentioned problems. This is achieved by the method of the type presented in the introduction, characterized in that the base station indicates in an information frame sent to the transcoder a synchronization error present in the information frames coming from the transcoder, and that the transcoder corrects its synchronization after receiving said message.
The invention also relates to a digital radio system comprising at least one base station communicating with terminals in its coverage area, and a mobile telephone exchange communicating with with the base station and arranged to control the operation of the base stations, and a transcoder unit arranged to code and decode information to be transmitted into a form suitable for the transmission, and that the base station is arranged to send information frames to the transcoder at a certain pace, and that the transcoder is arranged to send information frames to the base station at a certain pace. The system of the invention is characterized in that the base station is arranged to indicate in an information frame sent to the transcoder the synchronization error present in the information frames coming from the transcoder, and that the transcoder is arranged to correct its synchronization after receiving said message.
The solution of the invention provides many advantages. Both units (both the base station and the transcoder) can now correct synchronization errors. Frames are no longer lost in the uplink transmission direction because synchronization frames need not be sent.